6.1 - Expressing Reaction Rates

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16.1 - Expressing Reaction RatesReaction Rate – The change in amount of reactants or products over time.Rate of Reaction = Amount of Afinal – Amount of Ainitialtfinal – tinitialWhere, Amount is in moles and time (t) in secondsCan also be expressed:Rate of Reaction = ∆Amount of A , units are mol/s∆t

3Average Rate of ReactionThe rate of a reaction is not usually constantSee figure 3 (Pg. 362)Average Rate of Reaction – the average change in concentration per unit time over a given time interval.Slope of a line between 2 points on the curve of a Conc. Vs. Time graph

4Instantaneous Rate of ReactionThe rate of a reaction at a particular moment or instant in time. (ie. the rate at 10.0 seconds)The slope of a tangent to the curve on a Conc. Vs. Time Graph (Figure 4: Pg. 362)Tangent – touches the line at only one point.

5Why does Rate of Rxn Change?At the start,  [reactants] therefore more successful collisions occur and more product is formed.As the [reactants]  , there are fewer successful collisions occur so the rate of product formation .

6Calculating Rate of Reaction Using Molar RatiosGiven the following rxn:2N2O5(g)  4NO2(g) + O2(g)Rate of Rxn can be expressed as:Rate of disappearance of N2O5(g)Rate of production of NO2(g)Rate of production of O2(g)If we know the rate for one reactant/product we can use the molar ratio to calculate the others

10Methods for Measuring Rate of RxnDecrease in massIf a gas is produced...you must use an open containerChange in pHReactions involving acids and basesConductivityReactions involving ionic compounds in solutionPressure ChangesGases must be producedColour Change – increase or decrease over timeVolume – of a gas producedPractice: Pg. 365 #8